Communication networks are increasingly relying upon optical fiber as the transmission medium. Originally optical fiber was used as a point-to-point substitute for electrical transmission line, such as coaxial cable. An electrical data signal at the transmitting end of the fiber was used to modulate an optical source such as a laser emitting at a wavelength at which the fiber is substantially transparent. The optical fiber carries the modulated optical signal from the transmitting end to the receiving end. The receiver includes an optical detector which demodulates the optical signal to produce an electrical signal equivalent to the electrical data signal at the transmitting end. One disadvantage of this architecture is that while the bandwidth of silica optical fiber is measured in hundreds of terahertz (.about.10.sup.14 Hz), the system throughput is limited to the speeds of the electronic transmitters and receivers, currently approaching 10 Gb/s (10.sup.10 bits per second).
It was early realized that with proper components wavelength-division multiplexing (WDM) could multiply the throughput of an optical fiber. In WDM, at the transmitting end of the fiber, different data signals modulate optical sources emitting at different wavelengths. The multiple optical signals are multiplexed onto a single fiber. At the receiving end, the optical signals are demultiplexed into separate optical paths, and the separated optical signals are then individually detected. While WDM does multiply the fiber throughput by the number W of WDM channels, as described thus far, the architecture is still point-to-point. As a result, expensive opto-electronic converters as well as optical multiplexers and demultiplexers are needed at each node of the network, and these converters are very expensive and their designs are dependent upon data rate and signal format.
More recently, all-optical communication networks have been proposed that rely upon WDM and wavelength-selective switching. See for example Brackett et al., "A scalable multiwavelength multihop optical network: a proposal for research on all-optical networks," Journal of Lightwave Technology, vol. 11, pp. 736-753, 1993. One such all-optical network 10 is illustrated in the network diagram of FIG. 1. It includes four wavelength-selective cross-connects 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4 interlinked by unidirectional fiber cross-links 14.sub.1, 14.sub.2, 14.sub.3, 14.sub.4, 14.sub.5, 14.sub.6. In practice, the unidirectional links are almost always paired to produce a bidirectional link. However, the illustration is restricted for clarity to only a few unidirectional links. The all-optical network 10 interconnects multiple switch nodes 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4, 16.sub.5, which will be assumed to include W.times.W ATM switches, where W is the number of WDM channels. Unidirectional fiber in-links 18.sub.1, 18.sub.2, 18.sub.3, 18.sub.4, 18.sub.5 and unidirectional fiber out-links 20.sub.1, 20.sub.2, 20.sub.3, 20.sub.4, 20.sub.5 connect the cross-connects 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4 within the all-optical network 10 to the switch nodes 16.sub.1, 16.sub.2, 16.sub.3, 16.sub.4, 16.sub.5. An ATM switch is one conforming to the well known telephony standard of Asynchronous Transfer Mode.
A representative switch node 16 is schematically illustrated in FIG. 2. A fiber in-link 18 carrying up to W-WDM signals from the all-optical network 10 inputs to an optical demultiplexer 22 which separates the WDM signals into W separate paths. A detector array 24 has W detectors separately detecting the signals and producing corresponding electrical signals. A switch 26, here assumed to be W.times.W ATM switch, receives the electrical lines from the detector array 24 on electrical input ports 28. A complementary structure exists on the transmission side. A modulator array 32 of lasers receives W electrical signals from output ports 30 of the ATM switch 26 and converts those signals to optical signals on W optical links 33 having differing carrier wavelengths, and outputs the W optical signals to an optical multiplexer 34 which combines them to a single physical communication path. A fiber in-link 20 receives from the multiplexer 34 the multiplexed optical signals, that is, the WDM signal, and transmits it into the all-optical network 10. The ATM switch 26 is also connected by electrical add/drop lines to one, two, or more users 36, each of which may be a central office switch or other large user of bandwidth which further demultiplexes the signals for smaller users.
All of the fiber links discussed this far are assumed to be capable of carrying up to WWDM signals. An advantage of the all-optical network 10 is that, once a WDM signal is transmitted therein, it is routed according to the wavelength of the optical carrier wave. That is, a WDM wavelength channel is set up between a transmitting terminal and a receiving terminal, and a complex web of such wavelength channels may interconnect three or more terminals. An example of such an interconnection is illustrated in FIG. 3. Switching node 16.sub.1 transmits to switching node 16.sub.4 over WDM path 40.sub.1 while it transmits to switching node 16.sub.2 over WDM path 40.sub.2. The WDM path 40, includes the out-link 20.sub.1 (FIG. 1), the cross-connect 12.sub.2, the cross-link 14.sub.2 (FIG. 1), the cross-connect 12.sub.4, and the in-link 18.sub.4 (FIG. 1) while the WDM path 40.sub.2 includes the out-link 20.sub.1 (FIG. 1), the cross-connect 12.sub.2, and the in-link 18.sub.2 (FIG. 1). That is, the out-link 20.sub.1, carries both WDM paths 40.sub.1, 40.sub.2, but it is able to do this because the two WDM paths 40.sub.1, 40.sub.2 are assigned to different carrier wavelengths, and the wavelength-selective cross-connect 12.sub.2 directs the two wavelength-differentiated signals to the proper receiving node. Similarly, switching nodes 16.sub.2, 16.sub.3 establish bidirectional communication over WDM paths 42.sub.1, 42.sub.2. The WDM path 42.sub.1 extends over out-link 20.sub.2 (FIG. 1), cross-connect 12.sub.2, cross-link 14.sub.5 (FIG. 1), cross-connect 12.sub.3, and in-link 18.sub.3 (FIG. 1) while the WDM path 42.sub.2 extends over out-link 20.sub.3 (FIG. 1), cross-connect 12.sub.3, cross-link 14.sub.4 (FIG. 1), cross-connect 12.sub.2 and in-link 18.sub.2 (FIG. 1). This example importantly shows that the in-link 18.sub.2 is carrying two WDM signals from two different switch nodes 16.sub.1, 16.sub.3 over the WDM paths 40.sub.2, 42.sub.2. This can be accomplished only if the two WDM paths 40.sub.2, 42.sub.2 are assigned to different WDM wavelengths. That is, the network must be configured so that multiple WDM signals do not interfere on any link. Expressed differently, colors (wavelengths) cannot clash on any link.
The all-optical network 10 described above is best characterized as a mesh network. Other popular WDM network architectures include star networks and ring networks. The color clash problem addressed by the invention exists in all of these, and the solution presented by the invention can be applied to them all with minor modifications.
The operation of the all-optical network 10 depends on the cross-connects 12.sub.1, 12.sub.2, 12.sub.3, 12.sub.4 being able to switch signals among different outputs dependent upon the optical carrier wavelength of those signals. Such cross-connects are available in at least three different technologies. All these technologies advantageously allow the cross-connects to be reconfigured so as to, at different times, send signals of the same wavelength in different directions.
A first technology involves a hybrid WDM switch, such as described by Iqbal et al. in "High Performance Optical Switches For Multiwavelength Rearrangeable Optical Networks," Government Microelectronic Circuits Application Conference (GOMAC) '94, San Diego, Calif., Nov. 1994, 3 pp. This 4-wavelength 2.times.2 switch includes cascaded multilayer thin-film interference filters and discrete 2.times.2 cross-bar, relay-actuated optical switches connected to the outside with optical multiplexers and demultiplexers. The hybrid, fiber-based, WDM switch demultiplexes the input wavelengths to different 2.times.2 optical switches, and the mechanically switched signals are recombined (multiplexed) at the output. The relay-actuated opto-mechanical switches are mechanical switches in which fibers connected to the two inputs are mechanically moved into alignment with different output fibers. Although this technology is conceptually relatively unsophisticated, its subsecond switching speed is satisfactory for many envisioned architectures, it offers the assurance and reliability of a mechanical part, and is currently commercially available from JDS Fitel.
The second technology involves acousto-optical tunable filters (AOTFs), usually formed in a LiNbO.sub.3 substrate, in which the optical frequencies selected for switching are related to RF frequencies impressed on an interdigitated electrode formed over an optical waveguide in the piezo-electric LiNbO.sub.3 substrate, causing the selected signal to change its polarization state and thus to be switched by a polarization coupler. Such AOTFs are described by Cheung et al. in U.S. Pat. No. 5,002,349.
A third technology involves liquid-crystal switches, as described by Patel et al. in U.S. Pat. Nos. 5,414,540 and 5,414,541 and in "Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch," IEEE Photonics Technology Letters, vol. 7, 1995, pp. 514-516. By means of wavelength-dispersive layers and segmented liquid-crystal polarization-rotation layers, the different WDM channels can be switched in different directions.
It is anticipated that not all pairs of ATM-based switching terminals will always be interconnected by dedicated WDM paths. Dedicated WDM paths can be changed to single-hop connections if traffic is low or configuration problems prevent a direct WDM connection. Furthermore, a multi-hop connection can be formed for paths anticipated to carry little traffic. For example, as illustrated in FIG. 3, the ATM switch at node 16.sub.3 can transmit to the ATM switch at node 16.sub.1 through the ATM switch at node 16.sub.2, which after redirecting the ATM cell back onto the network 10 most likely wavelength converts the data signal for retransmission.
These considerations and the fact that substantial excess capacity will be available indicate that the reconfiguration problem can be best viewed as a topological problem of interconnecting the switching nodes. For example, as shown in FIG. 4, switching node 50.sub.1 has a transmitting link to switching node 50.sub.3 ; switching nodes 50.sub.2, 50.sub.3 are interconnected by anti-parallel links; and switching node 50.sub.4 has respective transmitting links to switching nodes 50.sub.1, 50.sub.3. This topological configuration T.sub.1 may reflect a daily traffic pattern, such as a network interconnecting financial institutions which, in the morning, have relatively heavy traffic. However, in the early afternoon, traffic subsides, and the topology T.sub.2 shown in FIG. 4 may be sufficient. Later in the evening, the traffic changes as the institutions exchange data and backup for the day's transactions. For this traffic, topology T.sub.3 illustrated in FIG. 4 may be optimal. At some time before the next morning's rush, the topology needs to change back to T.sub.1.
The concept of links needs to be separated from the call connections carried on those links. A call connection whether for voice, data, or video is anticipated to be of limited duration as determined by the nature of the call, a duration generally less than the time that a traffic topology is in place. After completion of the call, the call connection is terminated. However, in rearranging links, if a call connection is terminated on a link before the natural end of the call connection, the call is likely to experience loss of data, here called a hit. Even if the call connection is rerouted to another link, data may be lost during the link reconfiguration. It is assumed that the network control algorithm has no specific information about the intended duration of existing calls at the time a change of topology is desired.
Consistent with the switching speeds available in the different technologies available for wavelength-selective interconnects, it is anticipated that the switching configurations of the cross-connects be established for times that are much longer than the length of the electrical packet used for communication between the electrical switches. In the case of the switches being ATM switches, the length of the ATM cell is 125 .mu.s. Indeed, the configuration of the presently envisioned all-optical network is expected to change only on time periods of the order of hours. It is expected that the distribution of traffic density around the network changes sufficiently several times a day, as described immediately above, to justify reconfiguring the interconnects. A related time constraint involves the relative duration of the reconfiguration process compared to the duration of the traffic patterns prompting the reconfigurations. In order to simplify the problem to a single transition for the most part, it will be assumed that the time required to transition from the topological configuration T.sub.i to a new configuration T.sub.i+1 is shorter than the period over which the traffic pattern changes sufficiently to warrant a yet further configuration T.sub.i+2. This assumption does not rigorously follow from traffic patterns in a realistic network, but extra means will be incorporated to avoid problems associated with violations of the assumption.
However, reconfiguring operating networks introduces several difficulties. First, it is not desirable to break all existing call connections before the reconfiguration and to reestablish them on the reconfigured network. Although ATM cells for one connection can be routed along different paths, the cells can arrive out of order if the path has been reconfigured. Voice messages would be garbled during such a changeover.
A second problem in reconfiguring operating networks is that if the existing connections are broken before the reconfiguration process and then re-established after the reconfiguration, information will likely be lost. Any such loss will produce gaps in the voice, video, or data transmitted over the network to the users, an obviously undesirable characteristic of the network, particularly for data. The loss of cells or their garbling is referred to as a hit in the narrow sense. It is desirable to reconfigure a network with a minimum, preferably zero, of hits. In the broad sense, a hit occurs when a call connection needs to rerouted during network reconfiguration because the former link connection is disappearing.
Although the three network configurations described above for financial institutions are straightforward and amenable to manual determination, real-life networks are subject to more variables, such as week-ends, end of quarter, national holidays, local holidays, special events, emergencies, etc. Determining the proper network configurations for all these situations is a process that is preferably automated, and the transitions between such a large number of configurations need to be automated.